* Opposition seeking to overturn election results
* President calls for "multi-partisan process"
* Mahama says 2012 GDP growth 8.5 to 9 pct
By Kwasi Kpodo
ACCRA, Jan 4 (Reuters) - Ghana's president called for political unity on Friday, reaching out to rivals who are contesting his election due to suspicions of vote rigging.
John Dramani Mahama, a former vice president who took office in July after the death of President John Atta Mills, won a Dec. 7 election and is due to be sworn in on Monday along with a new parliament.
"For the long-term survival of our nation, we must agree and commit to a multi-partisan process," Mahama said in a speech to parliament. "Whatever our differences, whatever our politics, we must pull together and rise to meet these challenges."
International and local election observers said the December election - in which Mahama won 50.7 percent of the votes - was free and fair despite delays and technical problems that forced voting into a second day.
Ghana's main opposition party launched a legal challenge on Dec. 28, saying the poll involved enough irregularities to affect the outcome.
The opposition NPP party, whose leader, Nana Akufo-Addo, came second with 47.7 percent, has threatened to boycott Mahama's inauguration.
Ghana is one of Africa's fastest growing economies and has maintained three decades of peace, making it a favourite among international investors and an anomaly in a region better known for coups and civil wars.
Mahama said economic growth in the cocoa, oil and gold exporting nation was between 8.5 and 9 percent in 2012, but that political unity was required to ensure the rising productivity resulted in development.
Ghana became Africa's newest oil exporter in 2010 with the startup of Tullow Oil's offshore Jubilee field, propelling economic growth to 14.4 percent in 2011. Mahama said he expected 2012 growth to be between 8.5 and 9 percent.
"We have not only held down inflation and maintained macro-economic stability, but we have also worked to ensure discipline in the government's fiscal regime to avoid unbudgeted expenditures that could distort the economy's performance," Mahama said.
Inflation in Ghana has held under 10 percent. (Reporting by Kwasi Kpodo; Editing by Richard Valdmanis and Robin Pomeroy)